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46             - PRACTICAL ORGANIC  CHEMISTRY

If Wis the weight of salt, w the weight of silver, and ;/ the
basicity of the acid, the molecular weight of the silver salt is
determined4from the following- formula :—

The molecular weight of the acid is then obtained by deduct-
ing n atoms of silver and adding n atoms of hydrogen.

^Example— 0-3652 grm. silver benzoate gave 0*1720 grin.
silver.

108  X             - 108 + I = 122-2.

0'I72O

Calculated for C7H602 ; M = 122

Molecular Weight of Organic Bases
Determination by means of the Platinum Salt. —
The organic bases form, like ammonia, crystalline chloroplati-
nates with platinic chloride of the general formula B2H2,PtClfl.
By estimating the amount of platinum present in the salt, it is
possible to calculate the molecular weight of the platinum corn-
pound, and consequently that of the base.
Dissolve about i gram of an organic base (brucine, strych-
nine, guinine, &c.) in 10 c.c. of a mixture of equal volumes of
concentrated hydrochloric acid and water. To the clear hot
solution add excess of platinic chloride and let it cool. Yellow
microscopic crystals of the chloroplatinate of the base separate.
(If the chloroplatinate of the base is very soluble in water, such
as aniline, it must be washed with strong hydrochloric acid,
pressed on a porous plate and dried in a vacuum-desiccator over
solid caustic potash.)
Filter on the porcelain funnel with the pump and wash three
or four times with small quantities of cold water. Press the
precipitate down and dry on a porous plate in the vacuum -desic-
cator. When thoroughly dry, weigh out about 0-5 to i gram of
the compound into a porcelain or platinum crucible, and heat
gently with the lid on, and then more strongly until the organic
matter is completely burnt away. Cool the crucible in the desio
ea.or and weigh.